The greatest threat to the validity of meta-analysis is publication bias,
induced by the fact that
studies reporting a statistically significant result
are more likely to be published
[1]}.
Models dealing with publication bias have been extensively studied
in meta-analysis of intervention studies
and can be generally grouped into
graphical methods
or selection model based methods
[2]}.
Graphical methods,
such as the funnel plot and the trim-and-fill method,
are widely used to detect and adjust publication bias
[3]}.
Various kinds of sensitivity analysis methods are also available
based on Heckman-type
[4]}, [5]}
selection functions
[6]}, [7]}, [8]},
an alternative parametric selection function
[9]},
or
a worst-case nonparametric bound
[10]}.
